KSA hasn't got the best reputation, especially when it comes to baseball cards.
https://www.google.ca/search?q=ksa+g...hrome&ie=UTF-8
Personally, with me, and if I was in the market for one, I would likely avoid this one until I could have it verified by another TPG (PSA or SGC) or by someone who knows their stuff.
Out of curiosity, and only if you want to disclose it, what are they asking for this one?
There is some great info out there about spotting Mantle fakes and here are a few to help get you started.
Good luck.
http://bbcemporium.com/1952-topps-mi...terfeit-guide/
http://www.net54baseball.com/showthread.php?t=133602
https://sportscardinfo.wordpress.com...ey-mantle-311/